1. Field of the Invention
The present invention relates to a photo mask to be used for a lithography process for transferring a pattern onto a wafer in fabrication of a semiconductor device, and more particularly to a phase shift mask having a phase shift layer for generating a phase shift.
2. Description of the Prior Art
A phase shift type of mask is the mask used applied to obtain an improved lithography effect over the resolution and depth of focus produced by conventional lithography equipment by virtue of the phase difference between adjacent patterns. Much research on phase shift masks has been performed in many countries in order to obtain the lithography effect. Subsequent developments for the application of a phase shift mask to the fabrication of a semiconductor device are also expected.
Various types of phase shift masks have been known. They have advantages and disadvantages in accordance with their characteristics.
FIG. 1 is a sectional view illustrating a conventional Half-tone phase shift mask. FIG. 2A is a diagram illustrating the distribution of an electric field established ,just after light passes through the phase shift mask of FIG. 1. FIG. 2B is a diagram illustrating the distribution of the electric field established when the light passing through the phase shift mask of FIG. 1 reaches a wafer. FIG. 2C is a diagram illustrating the distribution of exposure energy established when the light passing through the phase shift mask of FIG. 1 reaches the wafer. FIG. 2D shows a sectional view of a photoresist film pattern formed on the wafer. A phase shift mask having a conventional structure will be described in brief, in conjunction with FIG. 1 and FIGS. 2A to 2D. In FIGS. 1 to 2D, the reference numeral 5 denotes a photoresist film pattern while the reference numeral 4 denotes a wafer.
As shown in FIG. 1, the conventional phase shift mask of the Half-tone type includes a quartz substrate 1, a chromium film 2 coated on the quartz substrate 1, and a phase shifter 3 made of a spin-on-glass (SOG) or polymethylmethacrylate (PMMA) coated over the chromium film 2. The phase shifter 3 may alternatively be interposed between the quartz substrate 1 and the chromium film 2. The chromium of the Half-tone phase shift mask serves to transmit light in some degree, for example, about 10%, as compared to existing masks and other types of phase shift masks. As the transmitted light passes through the phase shifter, it experience a phase shift of 180.degree. relative to light passing through peripheral patterns. As a result, an enhancement in depth of focus and resolution can be obtained. Such a Half-tone mask has a good future prospect because it is fabricated using the same pattern as that used for the existing pattern design and its fabrication is very simple.
In the case of the Half-tone mask shown in FIG. 1, light incident on the mask is decreased in strength and phase shifted by 180.degree. as it passes through a region where the chromium and phase shifter are disposed. On the other hand, light passing through only the quartz is not subjected to any decrease in strength and experiences a phase shift of 0.degree..
The distribution of the electric field established just after the light passes through the phase shift mask of FIG. 1 is shown in FIG. 2A. On the other hand, the distribution of the electric field established when the light passing through the phase shift mask of FIG. 1 reaches a wafer is shown in FIG. 2B.
FIG. 2C is a diagram illustrating the distribution of an exposure energy established when the light passing through the phase shift mask of FIG. 1 reaches the wafer. It is found that some degree of light strength is exhibited at opposite sides of a main waveform in a region where the phase of 0.degree. and the phase of 180.degree. meet together, due to light interference. Such light strength is increased if the chromium has a higher light transmittance. As a result, the enhancement in resolution and depth of focus to be obtained by the Half-tone characteristic is inhibited.
FIG. 2D is a sectional view illustrating a photoresist pattern formed on a wafer. It is found that the photoresist pattern has a ghost pattern 6. This ghost pattern 6 is formed by the above-mentioned undesirable light distribution. The depth of ghost pattern 6 increases as the light transmittance of the chromium is increased.
Consequently, where formation of a pattern is carried out using the above-mentioned conventional phase shift mask, there is a problem of formation of an undesirable pattern due to an undesirable light distribution caused by a light interference.